In attempting to dissect the various cellular events responsible for the development of EAE, we have studied the effect of in vitro culture with antigen on the ability of BP-sensitized cells to transfer EAE to naive recipients. Two different cell compartments, peritoneal exudate cells, PEC, and lymph node cells, LNC, display distingly different responses to BP in culture. In vivo, these two cells populations differ in the expression of cell surface markers (Ia antigens). PEC are Ia+ and LNC are Ia-. In contrast, after culture for 24 hours both populations are Ia+. They both appear to be activated in vitro by BP and fragments of BP which contain the GP encephalitogenic site.